Report first frame of animated image as LCP render time.

[chromium-wpt-export-bot]

Change-Id: If9df1402752265539fc82ae1b9ab356a7c1fceb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6038997
Reviewed-by: Chris Harrelson <[email protected]>
Commit-Queue: Ian Clelland <[email protected]>
Reviewed-by: Annie Sullivan <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1388807}

[wpt-pr-bot]

The review process for this patch is being conducted in the Chromium project.

[dev-ansung]

WPT Command: python3 ./wpt run --channel=nightly --verify --verify-no-chaos-mode --verify-repeat-loop=0 --verify-repeat-restart=10 --github-checks-text-file=/home/test/artifacts/checkrun.md --affected base_head --log-mach-level=info --log-mach=- -y --no-pause --no-restart-on-unexpected --install-fonts --no-headless --verify-log-full --binary=/home/test/build/firefox/firefox firefox

Some affected tests had inconsistent (flaky) results:

Unstable results

Test	Subtest	Results	Messages
/largest-contentful-paint/non-tao-image-load-after-fcp.tentative.html	Non-Tao Image Load and Render After FCP.	FAIL: 2/10, PASS: 8/10	assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 165 +/- 0.001 but got 164;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 208 +/- 0.001 but got 205
/largest-contentful-paint/non-tao-image-load-before-fcp-render-at-fcp.tentative.html	Non-Tao Image Load Before LCP and Render at the Same Time of FCP.	FAIL: 9/10, PASS: 1/10	assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 189 +/- 0.001 but got 180;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 106 +/- 0.001 but got 99;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 96 +/- 0.001 but got 95;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 188 +/- 0.001 but got 177;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 171 +/- 0.001 but got 160;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 157 +/- 0.001 but got 154;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 84 +/- 0.001 but got 83;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 110 +/- 0.001 but got 105;assert_approx_equals: LCP start time should be the same as FCP for blue.png expected 144 +/- 0.001 but got 129

These may be pre-existing or new flakes. Please try to reproduce (see the above WPT command, though some flags may not be needed when running locally) and determine if your change introduced the flake. If you are unable to reproduce the problem, please tag @web-platform-tests/wpt-core-team in a comment for help.

[dev-ansung]

created crbug.com/381331253
Please force merge in the meantime, thanks!

[jonathan-j-lee]

@KyleJu @DanielRyanSmith ^